1. Field of the Invention
This invention relates to polycarbonates and more particularly to polycarbonates having improved impact performance.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial applications because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for molding articles for which impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
It is known that polycarbonates exhibit high notched Izod (ASTM test D-256) impact values. These values, about 16 ft-lbs/in., associated with failure in a ductile mode, are characteristic of test specimens thinner than about one-eighth inch. Thicker specimens, i.e. one-fourth inch, fail in a brittle fashion at about 2.5 ft-lbs/in.
Impact strength dependence on thickness gives rise to "critical thickness", a material parameter defined as that thickness at which a discontinuity occurs in the graphical representation of impact strength vs. specimen thickness. For example, polycarbonate based on bisphenol A, having a melt flow rate of 3 to 6 grams/10 minutes at 300.degree. C. (ASTM D 1238) exhibits a discontinuity at about 5.71 mm (225 mils). Articles of this material thinner than 225 mils will show ductile failure upon impact while thicker ones will fail in a brittle fashion.
This dependence of impact performance on thickness represents a drawback because it tends to limit the utility of polycarbonates by imposing restrictions on the design of molded articles.
One approach to solving the critical thickness problem has been by incorporation of polyolefin into the polycarbonate resin (see U.S. Pat. No. 3,437,631). Another approach, taught by U.S. Pat. No. 4,043,980, entails the introduction of sulfur containing aromatic diols into the polycarbonate chain. Neither of these approaches is completely satisfactory as they tend to compromise some other important characteristics of polycarbonate resins. Acrylic rubber interpolymer composites have been suggested as impact modifiers of certain thermoplastic materials. These multiphase compounds have been disclosed in U.S. Pat. No. 3,808,180 as impact modifiers of vinyl halides and acrylic polymers and in U.S. Pat. No. 4,096,202 as modifiers of poly(alkylene terephthalates).